The present invention relates to a RFID tag read system and a RFID tag read method for making it possible to easily set the geometry of a read area in which a RFID tag becomes readable.
A RFID tag is so configured as to include an IC chip and a tag antenna connected to this IC chip. The RFID tag is classified into non-cell type including no cell, and cell type including a cell. An operation example of the non-cell type RFID tag is, for example, as follows: Namely, when a predetermined question radio wave is transmitted to the RFID tag by raising antenna of a reader, an answer radio wave for indicating identifier data stored in the IC chip is transmitted from the RFID tag. Moreover, the reader demodulates this answer radio wave. This process makes it possible to read the identifier data in a non-contact manner.
The non-cell type RFID tag extracts its operation power from a radio wave which the reader radiates thereto. Accordingly, remaining capacity of a cell will not exhaust out, and thus no life expectancy will expire. Also, the non-cell type RFID tag is not equipped with the bulky cell. Consequently, the RFID tag exhibits an advantage of being capable of implementing small size, light weight, and low price. In its operation power, however, the non-cell type RFID tag is dependent on the electromagnetic energy fed from the outside. As a result, causing the RFID tag to operate requires that the RFID tag be irradiated with an electromagnetic wave whose intensity is comparatively high.
In some cases, the reader of a RFID tag whose communications frequency band is higher than the UHF band, e.g., a 2. 45-GHz band, is equipped with patch antennas as the reader antenna for transmission/reception of the radio wave. In this case, dimension of the patch is equal to substantially 5 cm. Accordingly, it has been considered that the number of the patches be increased in order to enlarge the readable range. Increasing the number of the patches, however, has resulted in occurrence of the following problems: Namely, (1) the readable range becomes narrower, although gain enhances in a distant area. Also, the electromagnetic wave intensity in a proximate area becomes smaller, since high-frequency power fed depending on the number of the patches disperses. (2) The number of clearances between the patches increases, and thus these clearances increase the number of dead zones in the proximate area. On account of these problems, it has been wished to make it possible to easily set the geometry of a read area in which the RFID tag becomes readable.
Conventionally, there has been known the following “reader/writer antenna” (Refer to, e.g., JP-A-2004-199226 (paragraph [0018], FIG. 3)): Namely, a communications area is divided into four by four square loop antennas, and current phases of the respective loop antennas which implement the four divisions are shifted by 90°, respectively. This shift processing rotates resultant occurrence magnetic fields with frequency of driving power, thereby relaxing directivity specific to the loop antennas. Simultaneously, this shift processing enhances uniformity of radiation electric-field intensities of the four loop antennas, thereby decreasing a communications-incapable area due to direction of the tag. Simultaneously, this shift processing permits accomplishment of suppression of the electromagnetic radiation from the reader/writer antenna.
Also, there has been known the following “antenna device” (Refer to, e.g., JP-A-2004-266549 (paragraphs [0014] to [0015], FIG. 3)): Namely, first to third planar patch antennas are located on a side plane along displacement direction of a wireless tag such that the first to third patch antennas are arranged side by side in the up-and-down direction and on the forward side of displacement direction of the first and third planar patch antennas. This arrangement is made in order that the first to third patch antennas will not be overlapped with each other on the side plane. Simultaneously, the second planar patch antenna is located in proximity to the first and third planar patch antennas and on the backward side of the displacement direction. This location makes the first and third planar patch antennas and part of the second planar patch antenna look overlapped with each other when seen from the displacement direction of the wireless tag.